1. Field of the Invention
The present invention generally relates to a method of fabricating a silicon substrate having a thin buried silicon oxide layer (SOI substrate) and in particular to a method of this sort which makes it possible to produce an SOI substrate having a buried silicon oxide layer that is extremely thin and has excellent uniformity.
2. Description of the Related Art
The known techniques of fabricating an SOI substrate all have a number of drawbacks, in particular, a low production yield, a substrate quality which is still inadequate, and the production of a thin Si layer and a thin buried silicon oxide layer which are relatively thick and of mediocre uniformity.
A first method of fabricating an SOI substrate, known by the name of “SIMOX technology”, consists of forming the buried SiO2 layer in a silicon substrate by implanting oxygen at high dose followed by annealing at a temperature greater than 1300° C. A major drawback of this method is that it requires nonstandard equipment. Furthermore, the length of time for the process of implanting oxygen at high dose considerably reduces the production efficiency.
The substrates obtained by this method also suffer from inadequate quality of the buried silicon oxide layer and of the thin silicon layer (high pinhole density).
Finally, because the thin layers (thin Si layer and thin buried silicon oxide layer) are determined by the implantation process, this method makes it difficult to achieve thicknesses of less than 50 nm for the thin silicon layer and 80 nm for the buried SiO2 layer.
A second method, known by the name of “BESOI technique”, consists of forming a thin SiO2 film on a surface of a first silicon body, then bonding this first body to a second silicon body by means of the thin SiO2 film, and finally, removing part of one of the silicon bodies by mechanical grinding and polishing in order to form the thin silicon layer above the buried silicon oxide layer.
The silicon oxide layer on the first silicon body is formed by successively oxidizing the surface of this first body, then etching the oxide layer formed in order to obtain the desired thickness.
This method only allows relatively thick buried silicon oxide layers and silicon layers to be produced on the buried silicon oxide because of the poor control of the etching method. Furthermore, the thin layers obtained by this method have poor uniformity.
A third method, known by the name of “SMART CUT technology”, consists of forming, by oxidation, a thin silicon oxide layer on a first silicon body, then implanting H+ ions in the first silicon body in order to form a cavity plane in this first silicon body under the thin silicon oxide layer. Subsequently, by means of the thin silicon oxide layer, this first body is bonded to a second silicon body and then the assembly subjected to thermal activation in order to transform the cavity plane into a cleaving plane. This makes it possible to recover, on the one hand, an SOI substrate and, on the other hand, a reuseable silicon body.
This method requires the implantation of a high dose of hydrogen atoms. In spite of using atoms of smaller size for the implantation, the surface of the thin silicon layer obtained is also damaged. Furthermore, since the thickness of the thin silicon layer is defined by the implantation energy of the hydrogen atoms, it is difficult to make this thickness less than about 50 nm.
The methods above are described, in particular, in the article SOI: Materials to Systems, A. J. Auberton-Hervé, 1996, IEEE.
Therefore, a method of fabricating an SOI substrate, which overcomes the drawbacks of the methods of the prior art, may be desired.
In particular, a method of fabricating an SOI substrate which makes it possible to produce silicon oxide layers and silicon layers on the buried oxide layer that are very thin and of very good uniformity may be favorable.
A method of fabricating an SOI substrate that can be implemented in standard equipment may also be desired.